First, in order to increase the density and performance of VLSI (very large scale integrated circuits) integrated circuits such as memory circuits, the transistors disposed on the integrated circuit are made smaller and smaller. To further decrease transistor geometries below about 0.25 microns (gate length), advanced lithography tools including advanced steppers will likely be required. These tools are still being developed and are very expensive. It therefore is desirable to make smaller geometry transistors without using these advanced lithography tools.
Second, transistors may incorporate lightly doped drain regions in order to reduce hot carrier injection problems. FIG. 1 (Prior Art) is a simplified cross-sectional diagram of an N-channel transistor 1. To turn the transistor on, a positive voltage is applied to the gate 2. Electrons can then flow (due to the N+ type drain region 3 being positively biased with respect to the N+ type source region 4) from the N+ type source region 4, through the N- type lightly doped source region 5, through an inverted region 6 of silicon underneath the gate, through the N- type lightly doped drain region 7, and into the N+ type drain region 3. If lightly doped drain region 7 were not present, then the region between the drain region 3 and the region 6 would have a greater electric field (due to the bias voltage present between the drain and source regions). Accordingly, an electron traveling from region 6 and into drain region 3 would be accelerated by the electric field at the junction. This accelerated electron (called a "hot carrier") could then be injected into the gate oxide and become trapped in the gate oxide. This trapping of electrons degrades transistor performance.
In order to reduce the magnitude of the electric field, N- type lightly doped drain region 7 is provided so that the depletion region between regions 3 and 6 will be spread over a greater distance. It is generally true, however, that a double diffusion process is used to form lightly doped drain regions. As a result, an unwanted lightly doped source region is also formed when the desired lightly doped drain is being formed. Not only is the lightly doped source region not needed to reduce hot carrier injection effects, but the resistance added by the lightly doped source region increases the source-to-drain on-resistance of the transistor and therefore degrades transistor performance.